On Fri, Apr 27, 2012 at 5:58 AM, JMGross <msp...@grossibaer.de> wrote: > ----- Ursprüngliche Nachricht ----- > Von: Peter Bigot > Gesendet am: 26 Apr 2012 23:31:19 > >> The first draft of the design specification for 20-bit support in >> mspgcc has been added to the wiki at: > > Great news. Thanks for all your work. > > One thing I want to bring to your attention is a thread we had in > the TI E2E forum a few weeks ago. > The topic was a suspected compiler/processor bug when using > function pointers from an array. > > It turned out, that the CALLA instruction has a problem with the > straightforward approach with indexed addressing: > It isn't obvious, that the register part, which is predestined to hold > the variable index into the array, actually is the base address > (20 bit), whiel the immediate part, which is the only thing that > can be filled by the linker, is actually a _signed_ 16 bit index. > And when adding the two, the index is sign extended. > > So if the location of the (constant) array was above 32k, the > target address was fetched from 0xfxxxx instead of 0x0xxxx. > (of course a vacant memory address). > The immediate part of an indexed CALLA (or MOVA) > instruction therefore has only 15 usable bits, if it is meant > to be an (relocated) address. > Same is true for the MOVA instruction (and data pointers), > but there MOVX.A could be used instead.
I think that behavior is fairly clearly specified in the CPUX architecture documentation, but I can see that somebody might not be thinking about that when implementing the call operation in a higher level language. Thanks for highlighting the issue. Peter > As a result of the discussion, indexed use of CALLA should > be avoided. Better use MOVX.A to load the final address > into the register that holds the index (if optimization allows this) > and use CALLA in register mode. > > Apparently, nobody of the CCS compiler crew noticed this before. > (perhaps because all of their arrays during testing were in ram, > which is below 32k on all MSPs I know). > > JMGross > > ------------------------------------------------------------------------------ > Live Security Virtual Conference > Exclusive live event will cover all the ways today's security and > threat landscape has changed and how IT managers can respond. Discussions > will include endpoint security, mobile security and the latest in malware > threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ > _______________________________________________ > Mspgcc-users mailing list > Mspgcc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/mspgcc-users ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Mspgcc-users mailing list Mspgcc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mspgcc-users
